Volatile Organic Carbon Contaminated Site Assessment

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1 92 Volatile Organic Carbon Contaminated Site Assessment Introduction Roughly 75 percent of the major cities in the U.S. depend, at least in part, on groundwater for their water supply. Various estimates of the nationwide extent of groundwater contamination are stated to range from one to over two percent of the nation's usable groundwater (Council on Environmental Quality, 1981). Volatile organic compounds (VOCs) are the most frequently detected organic pollutants of groundwater in the United States. In fact, the VOCs are so ubiquitous that their analysis has been considered by the U.S. Environmental Protection Agency as a screening procedure to establish the need for more extensive characterization of groundwater samples from hazardous waste disposal sites. In the upstate region of New York (excluding Long Island), of approximately 570 groundwater contamination incidents reported by 1985, 98% involved either the volatile components of gasoline and petroleum or solvents and degreasers (NY State DEC, 1985). Volatile organics may be transported in the subsurface as dissolved components in groundwater. However, by virtue of their volatility, VOCs will also exist in the gas phase of unsaturated porous media. As a result, volatile contaminants can be transported by advection and diffusion in the vapor phase. VOC transport processes are illustrated in Figure 8-1. Experiment Description Students will use soil gas sampling to prospect for a VOC that has leaked from a subsurface source into an unsaturated soil system. A rectangular soil box contaminated with a combination of liquid acetone, octane and toluene will be used. A soil with high organic Spill, or Leaky Storage Tanks VOC movement as a Non Aqueous Phase Liquid (NAPL) Groundwater Flow VOC loss to the atmosphere Vadose Zone VOC movement as a solute dissolved in groundwater VOC movement as a gas in soil pores Figure 8-1. Subsurface VOC transport processes. The vadose zone is the region of the soil profile in which some pores contain gas and are therefore, unsaturated.

2 93 content (potting soil) or low organic content (sand) may be used as the box filling material (porous medium). The VOCs will be identified and measured using a gas chromatograph (GC). Experimental Procedures Calibration (Peak Times) Each compound will have a unique retention time in the gas chromatograph. The time for each of the 3 VOC peaks can be obtained by injection of 100 µl head space samples from crimp cap sealed vials containing a small volume liquid acetone, octane, and toluene. Use the syringe technique described below. Analyze each compound 4 times (12 samples) using a gas chromatograph (see for information on using the gas chromatograph). These analyses will also serve to calibrate the GC by generating the peak area that corresponds to the saturated vapor concentration. Gas chromatogram peak areas may be assumed to be directly proportional to the mass of vapor injected. Syringe technique for sampling vial headspace The purpose of this syringe technique is to minimize the effects of sorption to the Teflon and glass surfaces in the syringe and to eliminate carryover of sample in the needle. Using separate needles to collect samples and to inject into the GC eliminates needle carryover of sample. 1) Remove GC needle. 2) Purge syringe 5 times with room air to remove any residual VOCs. 3) Put on sample needle. 4) Insert into sample bottle (with syringe at zero volume) 5) Fill syringe fully with gas, wait 4 seconds, and purge syringe contents back into the source bottle (repeat 3 times). 6) Fill syringe and adjust to 100 µl. 7) Close syringe valve and remove syringe from sample vial and remove sample needle. 8) Put on GC needle. 9) Instruct GC to measure sample. 10) Insert needle in injection port, open syringe valve, inject sample, hit start button (or Enter) all as quickly as possible. 11) Remove syringe from the GC injection port. Soil Gas Sampling See Table 8-1 for physical properties of the VOCs. See Tables 8-2, 8-3, and 8-4 (in the Lab Prep Notes) for necessary reagents, equipment and GC method. Prior to the laboratory the instructor will create a spill of a VOC by injecting 10 ml of liquid of two or more NAPLs into the soil box to be sampled by students. During the lab Volatile Organic Carbon Contaminated Site Assessment

3 94 students will analyze approximately 50 soil gas samples from the soil box using the syringe technique outlined below. Results from the soil box analyses may be mapped using units of concentration (g/m 3 ). Syringe technique for soil gas sampling 1) Remove GC needle. 2) Purge syringe 10 times with room air to remove any residual VOCs. 3) Put on sample needle. Table 8-1. Aqueous solubility (g/m 3 ) Vapor Pressure (kpa) H (kpa m 3 /mol) HGL (g/l)/(g/l) Molecular Formula Molecular weight density (g/ml) 4) Insert into soil bed (with syringe at zero volume). 5) Fill syringe and adjust to 100 µl. Physical data for octane, acetone, Octane Acetone Toluene 0.6 very (1.47) (2.9) (CH 2 ) 6 CO C 6 H ) Close syringe valve, remove syringe from soil bed and remove sample needle. 7) Put on GC needle. 8) Instruct GC to measure sample (using software). 9) Insert needle in injection port, open syringe valve, inject sample, hit the enter key (or OK) all as quickly as possible. 10) Remove syringe from the GC injection port. Analysis of Soil Gas Sampling Students will use their analysis of VOC standards to obtain the corresponding GC retention times and use this information to identify the unknown VOCs in the contaminated soil box. The vapor pressure and ideal gas law are used to estimate the mass of each compound present in the samples used to calibrate the GC. PV n 8.1 RT where n is the number of moles of the compound, P is the vapor pressure of the compound [kpa], V is the syringe volume [L], R is the Gas Constant (8.31 [ L kpa] [ mol K] ), and T is the temperature of the gas in the syringe [K]. The relationship between peak area (as measured by the GC) and mass of the compound is determined from the calibration. Soil gas concentrations should be reported and plotted as contour lines on a map of the soil box (see Figure 8-2 for an illustration).

4 95 Procedure (short version) 1) Instructor will demonstrate syringe technique (be careful not to pull plunger out of barrel) and Gas Chromatograph technique. 2) Calibrate GC by analyzing 4 samples for each VOC. 3) Take soil gas samples. 4) Convert the soil gas peak areas to concentrations (g/m 3 ). This data will be shared between groups. 5) Clean plasticware. VOC NAPL source #1 Sand VOC NAPL source #2 Sample grids Figure 8-2. Students will prepare a map of the surface of their soil box. The map will show isoconcentration lines for each VOC. Prelab Questions 1) How are the identities of the chromatogram peaks determined when using a gas chromatograph? 2) Explain why different needles are used for sampling from source vials and injecting the sample into the GC. Consider the temperature of the injection port (see Table 8-4) and the fact that these compounds absorb to most surfaces. Data Analysis 1) Calculate the mass of each VOC in 100 L of headspace of vials containing liquid VOC. 2) Calculate the concentration of saturated vapor for each compound in g/m 3. 3) Plot isoconcentration lines of the identified VOCs (expressed as gas concentration in g/m 3 ) on maps of the contaminated site (see Figure 8-2 for example). Prepare a map for each compound showing isoconcentration lines. (The Excel 3-D surface plot with contour lines can be used. Note that the grid needs to have uniform distance between samples for the Excel 3-D surface plot to work correctly.) 4) Compare the saturated vapor concentration with the peak concentration observed in the sand box. References Ashworth, R. A., G. B. Howe, and T. N. Rogers. Air-Water Partitioning Coefficients of Organics in Dilute Aqueous Solutions. J. Hazard. Mater. 18, p , Council on Environmental Quality, "Contamination of Groundwater by Toxic Organic Chemicals", Hwang, Y., J. D. Olson, and G. E. Keller, II, Steam Stripping for Removal of Organic Pollutants from Water. 2. Vapor-Liquid Equilibrium Data. Ind. Eng. Chem. Res. 31, p , Volatile Organic Carbon Contaminated Site Assessment

5 96 Mackay, D. and W. Y. Shiu, A Critical Review of Henry s Law Constants for Chemicals of Environmental Interest, J. Phys. Chem. Ref. Data. 10, p , New York State Department of Environmental Conservation, "Draft Upstate New York Groundwater Management Program", N.Y.S.D.E.C., Division of Water, Draft Report WM P-94, January, 1985.

6 97 Setup 1) Prepare 1 soil box under fume hood. 2) Moisten the sand but not so much that there is standing water. Lab Prep Notes Table ) Pipette 10 ml of liquid acetone, octane, and toluene in sand box and record injection locations. This should be done in the morning before the lab exercise. Table ) Replace injection port septa on both GC s. 5) Verify that GC s are working properly by injecting gas samples from each VOC source bottle. If the baseline is above 30 (as read on the computer display) then heat the oven to 200 C to clean the column. 6) Verify that sufficient gas is in the gas cylinders (hydrogen, air, nitrogen). 7) Prepare VOC source vials that contain liquid acetone, octane, and toluene (they can be shared by two groups of students). Class Plan 1) Setup uniform spreadsheets for data entry Reagents list Description Source Catalog number Octane Fisher Scientific Acetone Fisher Scientific O299-1 toluene Fisher Scientific T Potting soil Agway (remove large particles by screening to 2 mm) Equipment list Description Supplier Catalog number 500 µl syringe w/ Supelco valve side port needle Supelco ml syringe w/ Supelco valve Hp 5890 Series II Hewlett-Packard 5890A GC Sep Hewlett-Packard option 600 purge-packed/fid 1/8" column Hewlett-Packard option 095 adapter pressure regulators Hewlett-Packard L43 RS232C board Hewlett-Packard option 560 Nitrogen, Air, and General Stores Hydrogen gas Wrist action Shaker Fisher Scientific Desiccator Fisher Scientific Vials Supelco Aluminum crimp Supelco tops Septa Supelco Crimping tool Supelco ) Make sure spreadsheet is completely filled out by end of lab Volatile Organic Carbon Contaminated Site Assessment

7 98 Table 8-4. Gas chromatograph conditions gas pressure flow carrier (N 2 ) 320 kpa 15 ml/min Air 230 kpa 300 ml/min Hydrogen 130 kpa 45 ml/min temperatures C oven (isothermal) 100 Injector 250 FID 250 Column Supplier Catalog number Supelcowax meters Supelco Run length of 66 seconds with octane, acetone, and toluene at 0.57, 0.63, 0.96 minutes respectively. Maximum sample volume is about 100 µl. Larger samples can lead to a significant broadening of the peak. Syringe clean up Disassemble and heat syringes to 45 C overnight to remove residual VOCs. Place syringe barrels upside down on top of openings above fan in oven to facilitate mass transfer.